The present invention relates to a steering angle ratio control system and method capable of varying the ratio of a steered angle of steerable road wheels to a steering angle of a steering wheel, i.e., a steering angle ratio by means of an electric driving mechanism such as an electric motor.
An electric power steering system adapted to assist a manual steering operation or effort by a power of an electric motor and a variable steering angle ratio steering system capable of varying the steering angle ratio have a possibility that a large current flows through an electric motor to overheat the electric motor if a steering wheel is held at a steering lock position or turned repeatedly to the steering lock position for a long time.
For carrying out a method of protecting an electric motor from such overheating, there has been, for example, an electric power steering control system that includes a torque sensor for detecting a steering torque of a steering wheel, an electric motor for assisting rotation of a steering shaft integral with the steering wheel, and a controller for controlling an operation of an electric motor in accordance with the steering toque. The controller estimates the temperature of the winding of the electric motor and decreases, when the temperature of the winding exceeds a certain limit, a counterelectromotive force constant that determines the angular velocity of the electric motor based on the temperature of the overheated winding thereby executing an electric motor temperature control (refer to Japanese Patent Provisional Publication No. 10-100913).
The above-described electric power steering control system can control the angular velocity of the electric motor and the current value for protecting the electric motor from overheating by decreasing the counterelectromotive force constant when the temperature of the winding of the electric motor estimated by the controller exceeds a certain limit value. The control system thus can prevent lack of torque similarly to the case of restricting the current value itself. However, by restricting the angular velocity, the responsiveness of the electric motor is lowered, thus making slower the response of the assist operation to the steering operation.
Further, in case the above-described electric motor protecting method is applied to a variable steering angle ratio steering system capable of varying the steering angle ratio by the operation of the electric motor, a steered angle of steerable road wheels that is varied depending upon a variation of a steering angle by a steering operation and an elapsed time needed for a variation of the steered angle in response to a variation of the steering angle is obtained as shown in FIG. 9. Namely, in case the electric motor has no restriction on the angular velocity but is supplied with a normal current value, a good responsiveness of the electric motor is obtained. For example, when the steering wheel is turned from a lock position for limiting its counterclockwise turning to another lock position for limiting its clockwise turning, an actual steered angle, as indicated by the line L1 in FIG. 9, is nearly the same as the target steered angle indicated by the line Lb, in response to the input angle (steering angle) indicated by the line La and therefore the actual steered angle can be responsive to the steering angle without deteriorating the responsiveness. However, in case the electric motor is restricted in the angular velocity, with the same load, thereby restricting the current value, a delay in the responsiveness of the electric motor is caused so that there is caused a delay of the actual steered angle shown by the dotted line L2 in reaching the target steered angle shown by the line Lb, thus causing a problem that a desired responsiveness cannot be attained.
It is accordingly an object of the present invention to provide a steering angle ratio control system and method capable of varying the steering angle ratio by means of an electric motor, which can protect overheat of the electric motor while maintaining a good responsiveness with which the steered angle can be varied in response to a variation of the steering angle.
According to an aspect of the present invention, there is provided a steering angle ratio control system comprising a steering angle sensor for detecting a steering angle of a manual steering operating device, a steered angle sensor for detecting a steered angle of steerable road wheels, a vehicle speed sensor for detecting a vehicle speed, a steering angle ratio varying mechanism having an electric drive unit and capable of varying a steering angle ratio by the electric drive unit, a temperature sensor for detecting the temperature of the electric drive unit, and a controller for controlling the steering angle ratio varying mechanism, the controller including a steering angle ratio determining section for determining a steering angle ratio based on the vehicle speed, a target steered angle calculating section for calculating a target steered angle of the steerable road wheels based on the steering angle ratio determined by the steering angle ratio determining section, and a steering angle ratio control section for controlling the steering angle ratio varying mechanism so that the steered angle of the steerable road wheels and the target steered angle coincide with each other, wherein the steering angle ratio determining section includes an electric drive unit overheat protecting section that determines a smaller steering angle ratio when the temperature of the electric drive unit rises beyond a predetermined temperature as compared with a steering angle ratio determined thereby when the temperature of the electric drive unit is lower than the predetermined temperature.
According to another aspect of the present invention, there is provided a steering angle control method comprising detecting a steering angle of a manual steering operating device, detecting a steered angle of steerable road wheels, providing a steering angle ratio varying mechanism having an electric drive unit and capable of varying a steering angle ratio by the electric drive unit, detecting a vehicle speed, determining a steering angle ratio based on the vehicle speed, calculating a target steered angle of the steerable road wheels based on the determined steering angle ratio, detecting the temperature of the electric drive unit, and controlling the steering angle ratio varying mechanism so that the steered angle of the steerable road wheels and the target steered angle coincide with each other, wherein the determining includes determining a smaller steering angle ratio when the temperature of the electric drive unit rises beyond a predetermined temperature as compared with that when the temperature of the electric drive unit is lower than the predetermined temperature.